Core-sheath filaments and production processes therefor are widely known. For instance, it is pointed out in EP-A-0 011 954 that special spinning equipment is required to avoid the occurrence of homofilaments even at a low sheath content. Despite the avoidance of homofilaments through the use of the known spinning equipment, it is impossible to avoid the presence in the resulting yarn of core-sheath filaments with a highly fluctuating sheath content, including sections without any sheath content, and a wide range of fluctuation of the sheath content of the core-sheath filaments within the resulting yarn.
Experiments have shown that in using a spinning apparatus as described in EP-A-0 011 954 and a feed of core and sheath material in a volume ratio of 85:15, as described therein in the Example, not more than 15% of the core-sheath filaments obtained in the yarn, generally even fewer, have a sheath content of about 15%, even if a sheath content fluctuation of .+-.10% is taken into account. The other core-sheath filaments in the yarn obtained have a higher (up to 30% by volume) or a smaller (down to below 5% by volume) sheath content.
Nor is it possible with the known process to obtain one or more homofilaments in the yarn in a controlled manner. The production of homofilaments is purely adventitious. There is no guarantee that a homofilament which is discernible in the yarn cross-section remains a homofilament in the yarn direction. On the contrary, viewed in the linear direction of the yarn, a homofilament will change into a core-sheath filament and vice versa.
The high fluctuation of the sheath content has the effect that every filament in the yarn has different properties. This means that the filaments in the yarn have widely differing properties, which is undesirable.
Basically, yarns composed of core-sheath filaments should have the desirable properties of a core material (strength, shrinkage, extension, birefringence, etc.) while the sheath improves the other properties of the yarn (adhesion to other materials, dyeability, fastness to handling, chemical and mechanical resistance, etc.). In existing processes, the average sheath content must be 20% by volume or more in order to keep the fluctuation of the sheath content within limits and to keep the properties of the core material uniform, to some degree, based on the total cross-section of the core-sheath filament.